| | Islip, NY | Reston, VA | United States |
| Population | 339,596 | 62,453 | 329,725,481 |
| Female Population | 50.0% | 51.9% | 50.5% |
| Male Population | 50.0% | 48.1% | 49.5% |
| Median Age | 38.7 | 39.6 | 38.4 |
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| Population - 2010 | 335,540 | 53,257 | 309,327,143 |
| Population - 2000 | 322,612 | 56,407 | 285,036,114 |
| Population - 1990 | 299,587 | 48,556 | 251,960,433 |
| Pop. 2000 to Now | 1.9% | 7.9% | 15.6% |
| Pop. 1990 to Now | 9.8% | 25.4% | 30.8% |
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| Population Density | 3,268.4 | 4,074.7 | 93.4 |
| Land Area | 103.9 | 15.3 | 3,531,905.4 |
| Water Area | 59.1 | 0.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Islip, NY | Reston, VA | United States |
| Age 0 to 4 | 6.0% | 5.6% | 5.9% |
| Age 5 to 9 | 5.9% | 6.3% | 6.1% |
| Age 10 to 14 | 6.5% | 4.9% | 6.6% |
| Age 15 to 17 | 4.0% | 2.7% | 3.9% |
| Age 18 to 20 | 3.5% | 2.4% | 4.0% |
| Age 21 to 24 | 5.4% | 4.7% | 5.2% |
| Age 25 to 34 | 13.7% | 16.3% | 13.8% |
| Age 35 to 44 | 13.3% | 14.2% | 12.9% |
| Age 45 to 54 | 14.3% | 13.1% | 12.6% |
| Age 55 to 59 | 7.2% | 7.1% | 6.7% |
| Age 60 to 64 | 6.6% | 6.9% | 6.3% |
| Age 65 to 74 | 8.0% | 9.4% | 9.6% |
| Age 75 to 84 | 3.9% | 5.2% | 4.5% |
| Age 85 and over | 1.8% | 1.1% | 1.9% |
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| Race | | | |
| | Islip, NY | Reston, VA | United States |
| White | 51.5% | 59.9% | 59.4% |
| Black | 8.0% | 8.9% | 12.2% |
| Asian | 3.6% | 11.7% | 5.6% |
| Native American | 0.1% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.6% | 0.7% | 0.4% |
| Two or More Races | 1.5% | 5.3% | 3.2% |
| Hispanic | 34.7% | 13.6% | 18.4% |
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| Marriage & Family | | | |
| | Islip, NY | Reston, VA | United States |
| Married Population | 49.6% | 53.1% | 49.8% |
| Currently Married | 47.6% | 51.5% | 48.0% |
| Married but Separated | 2.0% | 1.6% | 1.8% |
| Single Population | 50.4% | 46.9% | 50.2% |
| Never Married | 36.4% | 32.3% | 33.8% |
| Divorced | 8.8% | 11.2% | 10.8% |
| Widowed | 5.2% | 3.5% | 5.6% |
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| Household Size | 3.31 | 2.28 | 2.60 |
| Households | 101,741 | 27,366 | 124,010,992 |
| Family Households | 76,347 | 16,101 | 80,755,759 |
| Married couple, w/children | 33.5% | 31.0% | 30.5% |
| Married couple, no children | 40.4% | 47.4% | 42.9% |
| Non Family Households | 25,394 | 11,265 | 43,255,233 |
| Single householder, w/children | 12.8% | 12.0% | 15.8% |
| Single householder, no children | 13.4% | 9.5% | 10.8% |